Greenhopper upgrade fails : Could not load custom field

When upgrading to greenhopper 5.8.4, I get the stacktrace below.
I have a custom ranking field present.
What actions should I take?

011-12-14 09:22:45,526 main ERROR [jira.issue.managers.DefaultCustomFieldManager] Could not load custom field type plugin with key 'com.pyxis.greenhopper.jira:greenhopper-ranking'. Is the plugin present and enabled?
2011-12-14 09:22:45,541 main INFO [greenhopper.upgrade.task013.ConvertOldRankFieldsToNumberTypeTask] – Undo Done class com.atlassian.greenhopper.upgrade.task013.ConvertOldRankFieldsToNumberTypeTask
2011-12-14 09:22:45,557 main ERROR [sal.core.upgrade.PluginUpgrader] Upgrade failed: customField should not be null!
com.atlassian.plugin.util.Assertions$NullArgumentException: customField should not be null!
at com.atlassian.plugin.util.Assertions.notNull(Assertions.java:12)
at com.atlassian.greenhopper.upgrade.task013.CreateNewRankCustomFieldsFromOldTask.getNewlyInitialisedDefaultRankField(CreateNewRankCustomFieldsFromOldTask.java:88)
at com.atlassian.greenhopper.upgrade.task013.CreateNewRankCustomFieldsFromOldTask._doTask(CreateNewRankCustomFieldsFromOldTask.java:37)
at com.atlassian.greenhopper.upgrade.task013.CreateNewRankCustomFieldsFromOldTask._doTask(CreateNewRankCustomFieldsFromOldTask.java:23)
at com.atlassian.greenhopper.global.AbstractTransactionalTask.doTask(AbstractTransactionalTask.java:12)
at com.atlassian.greenhopper.global.TaskChain.execute(TaskChain.java:29)
at com.atlassian.greenhopper.upgrade.task013.GhUpgradeTask013.performUpgrade(GhUpgradeTask013.java:94)
at com.atlassian.greenhopper.upgrade.AbstractGhUpgradeTask.doUpgrade(AbstractGhUpgradeTask.java:46)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)